As a method for providing a shaft member made of a metal material as a plastic material with a desired stepped configuration, there is known a shaft enlarging working in addition to methods such as cutting, pressing or die forging (see, e.g., Patent Document 1). According to the working method disclosed in Patent Document 1, while rotating a shaft member, bending is applied to the shaft member while applying a compressive force to the shaft member in its axial direction. The compressive force and the bending plastically deform, that is, enlarge a portion of an outer circumferential surface of the shaft member so as to form the portion of the outer circumferential surface into an enlarged portion.
According to this enlarging method, differently from the cutting, swarf is not generated, and machining time is remarkably reduced. Further, as compared with a working machine for executing the pressing or the die forging, a working machine for executing the enlarging method can be downsized and working load can be reduced. Therefore, attention is being drawn to the enlarging method in recent years.